Over the past 25 years, transport activity has grown at approximately twice
the rate of energy efficiency improvements. Because the worlds transportation
system continued to rely overwhelmingly on petroleum as an energy source, transport
energy use and GHG emissions grew in excess of 2% per year. Projections to 2010
and beyond reviewed above reflect the belief that transport growth will continue
to outpace efficiency improvements and that without significant policy interventions,
global transport GHG emissions will be 50%100% greater in 2020 than in
1995. Largely as a result of this anticipated growth, studies of the technical
and economic potential for reducing GHG emissions from transport generally conclude
that while significant reductions from business-as-usual projections are attainable,
it is probably not practical to reduce transport emissions below 1990 levels
by the 20102015 time period. On the other hand, the studies reviewed generally
indicate that cost-effective reductions on the order of 10%20% versus
baseline appear to be achievable. In addition, more rapid than expected advances
in key technologies such as hybrid and fuel cell vehicles, should they continue,
hold out the prospect of dramatic reductions in GHG emission from road passenger
vehicles beyond 2020. Most analyses project slower rates of GHG reductions for
freight and air passenger modes, to a large extent reflecting expectations of
faster rates of growth in activity.

Assessing the total global potential for reducing GHG emissions from transportation
is hindered by the relatively small number of studies (especially for non-OECD
countries) and by the lack of consistency in methods and conventions across
studies. Not all studies shown in Table 3.16 cover the entire transportation
sector, even of the countries included in the study. Most consider a limited
set of policy options, (e.g., only motor vehicle fuel economy improvement).
In general, the studies do not report marginal costs of GHG mitigation, but
rather average costs versus a base case. Keeping all of these limitations in
mind, Table 3.16 summarizes the findings of several major studies. For 2010,
the average low GHG reduction estimate is just under 7% of baseline total transport
sector emissions in 2010, with the higher estimates averaging a 17% reduction.
There is, however, considerable dispersion around both numbers, indicative both
of uncertainty and differences in methodology and assumptions. For studies looking
ahead to 2020, the average low estimate is 15% and the average high estimate
is 34% of baseline 2020 transport sector emissions. Estimated (average rather
than marginal) costs are generally negative (as much as -US$200/tC), indicating
that fuel savings are expected to outweigh incremental costs. There are some
positive cost estimates as high as US$200/tC, however. The majority of the studies
cited in Table 3.16 are based on engineering-economic analyses. Some argue that
this method tends to underestimate welfare costs because trade-offs between
CO2 mitigation and non-price attributes (e.g., performance, comfort, reliability)
are rarely explicitly considered (Sierra Research, Inc., 1999).